Liquid chromatography has been a steadily growing technology for analytical chemists, and it is generally considered one of the most powerful analytical tools available due to its ability to separate, and thereby differentiate, compounds having minor stoichiometric or isomeric properties. In the field of liquid chromatography, one of the most useful instruments employed to identify sample materials is the spectrophotometer which are also known as photometric absorbance monitors and/or LC detectors. Spectrophotometers help identify compounds by measuring the absorbance by the sample material of light at different wavelengths. By measuring absorbance characteristics over a spectrum of wavelengths, the analysis can create a "fingerprint" of the sample material which may then be used to identify and/or characterize the material.
In order to accurately resolve a test sample compounds "fingerprint", a spectrophotometer must incorporate excellent optics and electronics design. Furthermore, it is desirable to have a spectrophotometer that can produce and monitor spectral absorbance over a wide range of wavelengths and include within that range a substantial portion in the ultraviolet spectrum. Moreover, it is desirable to measure absorbance at relatively narrow bandwidths selected over the wide spectral band. Also, it is desirable that such a spectrophotometer incorporate optics designed to minimize events which may inject error into the absorption measurement.
Prior absorption detectors in the field of liquid chromatography have typically either utilized the light source having a defined wavelength emission or, more preferably, a light source which emits radiation over a wavelength range. The light is then dispersed by means a grading into separate wavelength bands, and these bands are then selectively passed through the sample cell containing the material to be tested. Prior to passing a light beam through the sample cell, a portion of the beam is split to provide a reference to the intensity of the original beam so that this reference may be compared with the beam transmitted through the sample cell to determine the absorption of the selected wavelength of the beam. A discussion of such a detector is described in an article entitled "Design of an LC Detector: Part I", American Laboratory (May, 1987) co-authored by the present applicant. Furthermore, such detectors have been sold for a number of years.
In other devices, filters may be used to select a wavelength at which an absorption test is made. An example of such a device is described in one of my copending U.S. application, Ser. No. 08/093,065 filed Jul. 16, 1993 and entitled "Apparatus for Measuring Optical Absorption Properties of a Sample Material". U.S. Pat. No. 3,885,879 issued May 27, 1975 to Louder et al employs a dual beam spectrophotometer utilizing a movable spectral wedge to select a wavelength at which a test is made. The Louder et al reference also employs a bifurcated fiber optic bundle which defines a pickup for the test and reference wavelength band as well as a means for splitting the selected bandwidth between the sample cell and a reference.
A disadvantage of the design of existing spectrophotometers resides in the interaction of the optics with the physical properties of the sample cell. Typically, light is passed through the sample cell in such a manner that it impinges and reflects off of the cell walls before being sensed by the photodetector. Since test materials are typically dissolved in a carrier medium, the physical characteristics attendant the interface between the contact liquid layer and the cell sidewall alters the absorption properties of the cell sidewall so that error is introduced into the intensity measurement. That is, not all absorbance detected results from absorption of light by the test material. This can give a false profile or "fingerprint" of the test material and lead to either incorrect or inconclusive results.
Another disadvantage of many detectors is that the spectral gratings are typically optical elements having sufficient mass so that the inertia present during motion presents difficult mechanical drive problems. That is, it is difficult to start and stop the relatively heavy grating at each of the selected wavelength bands which can effect the resolution of the instrument.
In another of my co-pending U.S. applications, Ser. No. 08/344,209 filed Nov. 23, 1994 and entitled SPECTROPHOTOMETER APPARATUS, I disclose an improved spectrophotometer employing fiber optics to form an optical slit and describing an improved sample cell and focusing structure for the sample material to be tested. This application also discloses an improved arrangement of optical elements and electro-mechanical drives therefor in an effort to increase performance of the spectrophotometer.
Despite the advances evidenced by the above-described technologies, there remains a continuing need for improved spectrophotometer designs which can produce tests results having reduced error. Moreover, there is a continued need for higher resolution apparatus which can accurately and precisely measure optical absorption over a relatively narrow bandwidth. It is a need for such spectrophotometers, of course, to be able to provide measurements over a wide range of available wavelengths while maintaining such narrow bandwidth, especially in wavelengths including the visible, near-infrared and ultraviolet frequencies. A further need exists for simplified construction of such spectrophotometers.